1. Field of the Invention
The present invention relates to a DC to DC converter for raising a voltage supplied from a voltage-generating source and retaining a raised voltage in an arbitrary polarity.
2. Description of the Related Art:
In general, DC to DC converters are classified into a chopper type switching converter, a flyback converter, a forward converter, a charge pump type converter, etc. These types are used in different ways depending on the purpose of use.
The respective types of DC to DC converters may be compared with each other as follows. The chopper type switching converter requires a coil. The flyback converter and the forward converter require a transformer. Therefore, these types are disadvantageous when miniaturization is required, and are expensive. Further, the circuit configuration is complicated as well, and the adjusting operation is also troublesome.
On the other hand, the charge pump type converter requires no large parts such as a coil or a transformer. Therefore, this type converter is advantageously miniaturized, and the circuit can be constituted inexpensively.
A mechanical vibration-electric energy converter, in which a DC to DC converter 200 of the charge pump type is applied, will now be explained with reference to FIGS. 25 to 27.
This converter is reported in a literature xe2x80x9cA Micropower Programmable DSP Powered using a MEMS-based Vibration-to-Electric Energy Converterxe2x80x9d (Rajeevan Amirtharajah et al., M.I.T., 2000 IEEE International Solid-State Circuits Conference).
As shown in FIG. 25, the DC to DC converter 200 comprises a pump capacitor Cp, a reservoir capacitor Cr, an inductor L, and a plurality of switching elements SW1, SW2. Specifically, a first series circuit 202 including the pump capacitor Cp and the reservoir capacitor Cr connected in series, and a second series circuit 204 including the first and second switching elements SW1, SW2 connected in series are connected to one another in parallel. A connection point p1 of the pump capacitor Cp and the reservoir capacitor Cr of the first series circuit 202, and a connection point p2 of the first and second switching elements SW1, SW2 of the second series circuit 204 are connected via the inductor L. Further, a parasitic capacitor Co is connected in parallel to the pump capacitor Cp. A load 206 is connected in parallel to the reservoir capacitor Cr.
As shown in FIG. 26, the pump capacitor Cp comprises a comb-shaped movable electrode 210 which is arranged at the center, and comb-shaped fixed electrodes 212 which are fixed on both sides of the movable electrode 210. The distance d between the movable electrode 210 and the fixed electrode 212 is changed when their comb teeth 210a, 212a make approach to or make separation from each other. Thus, the capacitance is variable.
The operation of the DC to DC converter 200 shown in FIG. 25 will be explained with reference to a timing chart shown in FIG. 27. At first, the pump capacitor Cp has the maximum value of the capacitance when the fixed electrode 212 and the movable electrode 210 make approach most closely to each other. It is assumed that the electric charge is stored in the reservoir capacitor Cr with its terminal voltage of Vdd, for example, and no electric charge is stored in the pump capacitor Cp and in the parasitic capacitor Co respectively. Further, both of the first and second switching elements SW1, SW2 are in the OFF state.
At the start of an interval t1, when the second switching element SW2 is turned ON, a ramp current (inductor current iL) flows from the reservoir capacitor Cr to the inductor L in the interval t1. At the start of a next interval t2, when the first switching element SW1 is turned ON, and the second switching element SW2 is turned OFF, then the inductor current iL is supplied to the pump capacitor Cp in accordance with the energy of the inductor L in the interval t2, and the electric charge is stored in the pump capacitor Cp. In accordance with the storage of the electric charge, an output voltage Vc becomes a voltage (VSTART+Vdd) obtained by adding Vdd to the terminal voltage (start voltage VSTART) obtained when the capacitance of the pump capacitor Cp has the maximum value. The change to the voltage (VSTART+Vdd) follows the transient characteristic depending on the time constants of the pump capacitor Cp and the inductor L.
Subsequently, at the start of an interval t3, when both of the first and second switching elements SW1, SW2 are turned OFF, then the fixed electrode 212 and the movable electrode 210 of the pump capacitor Cp are controlled in the direction to make gradual separation from each other in the interval t3, and the capacitance of the pump capacitor Cp is gradually decreased. In accordance with the change of the capacitance, the output voltage Vc is gradually increased. The interval t3 comes to end at the point of time when the capacitance of the pump capacitor Cp is minimum, and then a next interval t4 is started. At the end of the interval t3, the output voltage Vc becomes a voltage (Vmax+Vdd) obtained by adding Vdd to the terminal voltage (maximum voltage Vmax) obtained when the capacitance of the pump capacitor Cp has the minimum value.
At the start of the interval t4, when the first switching element SW1 is turned ON, the current flows in the interval t4 from the pump capacitor Cp to the inductor L. At the end of the interval t4, the output voltage Vc becomes Vdd. The change of the output voltage Vc from the voltage (Vmax+Vdd) to the voltage Vdd follows the transient characteristic depending on the time constants of the pump capacitor Cp and the inductor L. However, the voltage arrives at the voltage Vdd for a short period of time as compared with interval t2, because the capacitance of the pump capacitor Cp is minimum.
At the start of a next interval t5, when the first switching element SW1 is turned OFF, and the second switching element SW2 is turned ON, then the energy stored in the inductor L is transmitted to the reservoir capacitor Cr in the interval t5. That is, the energy (energy generated by the pump capacitor Cp), which has been increased owing to the increase in voltage in the intervals t4, t5, is recovered by the reservoir capacitor Cr.
In order to increase the capacitance change of the pump capacitor Cp in the DC to DC converter 200 described above, the following artifices are required.
(1) The gap between the comb teeth 212a of the fixed electrode 212 and the comb teeth 210a of the movable electrode 210 is decreased.
(2) The thicknesses of the fixed electrode 212 and the movable electrode 210 are increased.
(3) The lengths of the respective comb teeth 212a, 210a of the fixed electrode 212 and the movable electrode 210 are increased.
(4) The numbers of the respective comb teeth 212a, 210a of the fixed electrode 212 and the movable electrode 210 are increased.
However, when the comb teeth 212a, 210a are formed, the isotropic etching such as the wet etching is used. Therefore, when it is intended to decrease the gap between the comb teeth, it is necessary to regulate the etching depth. Then the thicknesses of the fixed electrode 212 and the movable electrode 210 are decreased, and the electrode area is decreased. In such a situation, it is impossible to expect the effect (increase of the capacitance change) to be brought about by decreasing the gap. That is, the artifices (1) and (2) are in a relation of trade-off.
The increase of the vibration frequency of the movable electrode 210 contributes to the raising of the output voltage Vc. However, if the lengths of the respective comb teeth 212a, 210a of the fixed electrode 212 and the movable electrode 210 are increased, or if the numbers of the comb teeth 212a, 210a are increased, then it is impossible to increase the vibration frequency of the movable electrode 210. That is, the artifices (3) and (4) and the vibration frequency of the movable electrode 210 are in a relation of trade-off.
Further, if the thicknesses of the fixed electrode 212 and the movable electrode 210 are increased, or if the numbers of the respective comb teeth 212a, 210a of the fixed electrode 212 and the movable electrode 210 are increased in the situation that the gap between the comb teeth cannot be decreased so much as described above, then the size of the pump capacitor Cp itself is consequently increased. That is, the artifices (2) and (4) and the miniaturization are in a relation of trade-off.
In the DC to DC converter 200 described above, the air intervenes between the fixed electrode 212 and the movable electrode 210. Further, the capacitance is changed by only the change of the distance d between the fixed electrode 212 and the movable electrode 210. Therefore, it is impossible to effectively increase the capacitance change.
The present invention has been made taking the foregoing problems into consideration, and an object thereof is to provide a DC to DC converter which makes it possible to effectively increase the capacitance change for raising the voltage and which makes it possible to increase both of the output voltage and the voltage-raising ratio even when precise machining is not performed.
The present invention lies in a DC to DC converter comprising a voltage-generating source; a first element for raising a voltage supplied from the voltage-generating source based on capacitance-varying operation performed by an actuator section; and a second element for retaining a voltage after being raised by the first element in an arbitrary polarity, wherein the first element includes a capacitance-forming component and the actuator section, and the capacitance-forming component includes a first electrode section connected to a current supply line, a second electrode section installed in the actuator section, and a dielectric member arranged between the first and second electrode sections.
Accordingly, at first, when the voltage from the voltage-generating source is supplied to the first element, then the actuator section of the first element is driven, and thus the distance between the first electrode section and the second electrode section is changed. As a result, the contact area of the dielectric member with the respective electrode sections is also changed.
It is now assumed that the distance between the electrode sections is represented by d, the contact area of the dielectric member with the respective electrode sections is represented by S, and the dielectric constant of the dielectric member is represented by ∈. Then, a capacitance C is represented by the following expression.
C=∈S/d
In the conventional DC to DC converter, the capacitance has been changed by changing only the distance between the electrode sections. However, in the present invention, not only the distance between the electrode sections but also the contact area of the dielectric member with the respective electrode sections are changed. Therefore, it is possible to increase the capacitance change.
The dielectric member other than the air intervenes between the electrode sections. Therefore, it is unnecessary to form any precise gap between the respective electrode sections. It is possible to mitigate the various types of relations of trade-off. Thus, it is possible to effectively increase the capacitance change.
In the DC to DC converter constructed as described above, it is also preferable that the actuator section includes an operating section, a vibrating section for supporting the operating section, and a fixed section for vibratingly supporting the vibrating section, and the operating section includes a shape-retaining layer and at least a pair of electrodes to which a driving voltage is applied formed on the shape-retaining layer.
It is also preferable that the vibrating section and the fixed section are integrally formed of ceramics, and the shape-retaining layer comprises a piezoelectric/electrostrictive and/or anti-ferroelectric layer. In this configuration, it is also preferable that one electrode of the pair of electrodes of the operating section also serves as the second electrode section installed in the actuator section of the capacitance-forming component. Accordingly, it is possible to simplify the structure. Further, it is also possible to improve the driving efficiency of the actuator section. In other words, it is possible to adopt the structure in which the shape-retaining layer is interposed between the one electrode and the other electrode. The strain of the entire actuator section can be used for the displacement by applying the electric field to contribute to the strain over the entire actuator section.
It is also preferable that an insulating layer is allowed to intervene between one electrode of the pair of electrodes of the operating section and the second electrode section installed in the actuator section of the capacitance-forming component. That is, the insulating layer intervenes between the capacitance-forming component and the actuator section. The electric potential of the electrode of the actuator section can be set irrelevant to the capacitance-forming component. Therefore, it is possible to maximize the displacement obtained by applying an optimum driving voltage for the actuator section.
It is preferable that the dielectric member is made of a member having elasticity. Accordingly, it is possible to efficiently change the distance between the first electrode section and the second electrode section of the capacitance-forming component and the contact area of the dielectric member with the respective electrode sections, respectively, by driving the actuator section. It is possible to increase the capacitance change in the capacitance-forming component. Especially, when a ferroelectric filler is contained in the member, it is possible to increase the dielectric constant of the dielectric member, and it is possible to further increase the capacitance change.
It is also preferable that a member having a dielectric constant different from a dielectric constant of the dielectric member and having fluidity is arranged at least around the dielectric member. Accordingly, when the dielectric member is separated from the first electrode section of the capacitance-forming component in accordance with the driving action of the actuator section, for example, the member having fluidity flows into a formed gap. The dielectric member and the member having fluidity intervene between the first electrode section and the second electrode section. As a result, the dielectric constant between the first electrode section and the second electrode section is changed. It is possible to further facilitate the increase of the capacitance change by the change of the dielectric constant.
In the present invention, it is also preferable that the DC to DC converter further comprises a first switching element for selectively introducing the voltage from the voltage-generating source to the first element based on a first control signal; and a second switching element for selectively introducing the voltage after being raised by the first element to the second element based on a second control signal.
In this configuration, it is also preferable that the DC to DC converter further comprises a third switching element for selectively supplying a reference voltage and a first voltage different from the reference voltage to the actuator section based on a third control signal.
Accordingly, for example, when the voltage from the voltage-generating source is introduced into the first element by the first switching element, the capacitance of the capacitance-forming component is changed in accordance with the displacement action of the actuator section by supplying, for example, the first voltage to the actuator section by the third switching element. The voltage supplied from the voltage-generating source is raised based on the capacitance change. After that, the raised voltage is introduced into the second element by the second switching element. For example, the voltage from the voltage-generating source can be used for the first voltage different from the reference voltage.
In the DC to DC converter constructed as described above, it is also preferable that the third switching element becomes a stopped state when the voltage retained by the second element arrives at a predetermined voltage, and the DC to DC converter further comprises a fourth switching element for selectively supplying the reference voltage and a second voltage different from the reference voltage to the actuator section based on a fourth control signal. In this configuration, it is also preferable that the voltage retained by the second element is used for the second voltage different from the reference voltage.
Accordingly, it is possible to provide a plurality of stages of the change, i.e., the capacitance change based on the first voltage and the capacitance change based on the second voltage as the capacitance change in the actuator section. Further, the voltage obtained at the voltage-generating source and the voltage retained by the second element may be used as the first and second voltages. Therefore, it is unnecessary to provide any power source circuit system for newly generating the voltage. It is possible to simplify the circuit configuration and the apparatus configuration.
In the present invention, it is also preferable that each of the switching elements comprises a piezoelectric relay having a switching actuator section, and the switching actuator section includes a shape-retaining layer, an operating section having at least a pair of electrodes formed on the shape-retaining layer, a vibrating section for supporting the operating section, and a fixed section for vibratingly supporting the vibrating section.
Accordingly, it is possible to decrease the ON resistance of the switching element. Further, it is possible to realize the high speed switching operation. Therefore, it is possible to obtain the DC to DC converter having a compact size, a high output, and a high efficiency.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.